The present invention relates to a method (and apparatus) for reducing the emissions of nitrous oxides N.sub.2 O to the atmosphere from the combustion of nitrogen containing fuels or other nitrogen containing combustible compounds. More particularly, this invention relates to a method and apparatus for reducing such emissions when combusting solid fuels or the like in fluidized bed reactors.
As is well known, oxides of nitrogen are expelled to the air mainly from traffic, energy production (e.g. coal combustion), and waste management. Various oxides of nitrogen are produced during the combustion of most fuels with air. These nitrogen oxides result either from the oxidation of nitrogen in the air itself at elevated temperatures of combustion or from the oxidation of nitrogen contained in the fuel.
Numerous attempts have been made to develop methods which reduce the nitrogen oxide emissions in furnaces. The efforts have especially been towards the reduction of nitrogen dioxide (NO.sub.2) emissions in flue gases.
Another oxide, nitrous oxide N.sub.2 O, has recently been discovered to be one of the "greenhouse effect gases" that is increasing in the atmosphere and may contribute to global warming. When oxidizing in the upper tropospherical layers nitrous oxides (N.sub.2 O) generate nitric oxide NO which is considered to be one of the most important air pollutants: EQU N.sub.2 O+hv=N.sub.2 +O N.sub.2 O+O=2NO
Nitric oxide has a similar effect on the climate as carbon dioxide, potentially increasing the temperature and destroying the ozone layer.
It has been reported that N.sub.2 O emissions are generated in higher degree in combustors with relatively low combustion temperatures such as 750.degree.-900.degree. C. At higher temperatures the formation of N.sub.2 O does not seem to be a problem, as the formation of N.sub.2 O is slow and the reduction of N.sub.2 O to N.sub.2 is high.
Fluidized bed combustors operate at temperature ranges more favorable for N.sub.2 O formation, than most other types of combustors. N.sub.2 O emissions from circulating and bubbling fluidized bed boilers may be on the level of 50-200 ppm, higher than desired. The object of this invention is, therefore, to provide a method of reducing the emission of N.sub.2 O both from atmospheric and pressurized circulating or bubbling fluidized bed boilers.
The invention is based on the understanding of the kinetics of the formation and destruction of N.sub.2 O. It has been suggested, that HCN, which can be formed from volatile nitrogen or char nitrogen, is the major precursor of N.sub.2 O formation in combustors, and that N.sub.2 O reduction is strongly dependent on the temperature or H radical concentration. The increase in temperature or H radical concentration promotes N.sub.2 O reduction via the reaction EQU N.sub.2 O+H.fwdarw.N.sub.2 +OH.
Kramlich et al (Combustion and Flame 77:p.375-384, 1989) have made experiments in order to study the N.sub.2 O formation and destruction in a tunnel furnace, which was fired on either natural gas or oil. Nitrogen-containing compounds, such as HCN and acetonitrile, were doped into the flow. According to Kramlich et al maximum N.sub.2 O emissions of about 245 ppm occurred at 977.degree.-1027.degree. C. for HCN addition and of about 150 ppm at 1127.degree.-1177.degree. C. for acetonitrile addition. The study also showed that N.sub.2 O concentration was reduced from 240 ppm to 10 ppm by increasing the tunnel furnace temperature to over 1200.degree. C. during HCN injection into the furnace or to over 1300.degree. C. during acetonitrile injection, i.e. relatively high temperatures were needed according to this study.
Kramlich et al also studied the influence of NO.sub.x control on N.sub.2 O emissions. Especially the reburning of a portion of the fuel by fuel staging in the tunnel furnace was studied. In reburning, a portion of the fuel is injected after the main flame zone, which drives the overall stoichiometry to a fuel-rich value. After a certain time in the fuel-rich zone, air is added to fully burn out any remaining fuel. Kramlich et al discovered that reburning of coal in a second stage increases N.sub.2 O emissions whereas reburning of natural gas in the furnace exerts an opposite influence to that of coal and destroys N.sub.2 O.
It is an object of the present invention to provide a simple and economical method and apparatus for the reduction of N.sub.2 O emissions from atmospheric and pressurized circulating and bubbling fluidized bed boilers.
It is further an object of the present invention to provide a method and apparatus for creating favorable conditions for the destruction of nitrous oxides N.sub.2 O contained in flue gases discharged from fluidized bed combustors.
It is still further an object of the present invention to provide a method for reduction of N.sub.2 O in flue gases which can easily be retrofitted into existing fluidized bed combustion systems without interfering with existing processes.
In accordance with the present invention there is provided a method of reducing emissions of N.sub.2 O in flue gases from the combustion of nitrogen containing fuels in a fluidized bed reactor. A first combustion stage is arranged in a fluidized bed of particles. Fuel and an excess of an oxygen-containing gas at an air coefficient &gt;1 may be introduced into a first combustion stage for combustion of the fuel (i.e. oxygen-containing gas may be injected into the first combustion stage in an amount to generate flue gases containing residual oxygen). A temperature of about 700.degree. C.-900.degree. C. is maintained in the first combustion stage. The flue gases containing residual oxygen are conveyed from the first combustion stage into a flue gas passage. An additive selected from a group of chemical compounds able to form hydrogen (H) radicals is injected into the flue gas passage in order to generate sufficient quantities of hydrogen radicals to promote the reduction of N.sub.2 O in the flue gases. Preferably the additive injected is combusted to provide combustion heat for raising the temperature of the flue gas passage to &gt;900.degree. C., preferably to about 950.degree.-1100.degree. C. The group of additives able to form hydrogen radicals comprise compounds such as alcohol or natural gas, or other hydrocarbon gases such as liquefied petroleum gas or gasifier or pyrolyser gas, or oil. A good mixing between the flue gas and the formed hydrogen radicals is provided by injecting the additive at a location where a good mixing is easily arranged or is already prevailing in the flue gas flow. Good mixing facilitates the reactions between N.sub.2 O and H radicals. The amount of additive injected is adapted to the amount of N.sub.2 O in the flue gases.
The present invention is especially applicable when combusting solid fuels or waste materials in fluidized bed combustors at temperatures below 900.degree. C. The solid fuel or waste is introduced into the fluidized bed where--due to good mixing with the fluidized particles--it almost immediately reaches the bed temperature and is combusted. Temperatures in fluidized beds are normally between 700.degree.-900.degree. C. which gives optimal conditions for the combustion itself and, e.g., sulphur reduction in the flue gases. NO formation is low due to the relatively low combustion temperature, but N.sub.2 O may be formed.
In circulating fluidized beds the velocity of the fluidizing air is high enough to entrain a considerable amount of the bed particles out from the combustion chamber with the flue gases. The particles entrained are separated from the flue gases and recycled to the combustion chamber through a recycling duct. The circulation of particles from the combustion chamber through the particle recycling path back to the combustion chamber brings about a uniform temperature in the entire system which leads to more efficient combustion and longer residence times in the system as well as improved sulphur capture from flue gases.
Unfortunately N.sub.2 O formation seems to be facilitated by the low temperatures used in both bubbling and circulating fluidized beds. According to the present invention the N.sub.2 O concentration in the flue gases can be decreased by the injection of an additive capable for forming hydrogen radicals at the flue gas temperature and/or by slightly increasing the temperature of the flue gases.
The types of additives (e.g. additional fuels) which can be injected into the flue gas flow in order to reduce N.sub.2 O concentration include:
natural gas or methane,
liquefied petroleum gas,
oil,
alcohol, e.g. methanol or ethanol,
gas from a pyrolyser or gasifier, or
any gaseous, liquid or solid fuel, having a hydrogen component, and a heat value of at least 1 MJ/kg.
Gases may be introduced through gas inlet nozzles without any carrier medium, or with an oxygen containing gas. Oil or fine solid fuel may be introduced with carrier gas such as air or recycled flue gas.
The additives injected into the flue gases are preferably injected at a location separate from the first combustion stage in order not to interfere with reactions taking place there. Preferably the additives should not be injected so that they significantly increase the temperature of the fluidized bed particles.
In order to ensure effective reduction of N.sub.2 O the additive should be injected at a location where the whole flue gas flow can easily be affected by the introduction of the additive. The temperature of the whole flue gas flow should be increased and/or hydrogen radicals formed should come into contact with the whole flue gas flow in order to achieve a maximum reduction of N.sub.2 O.
The additive or additional fuel may be injected into the following locations:
a section of the fluidized bed combustor, or elsewhere, where bed density is less than 200 kg/m.sup.3,
a duct between the combustion chamber and a cyclone or other gas particle separator,
a cyclone or other gas particle separator itself, in any number of configurations,
ducts between two cyclones or other gas particle separators, or combination thereof connected in series,
any location in the backpass after the combustor and before a stack or gas turbine, or
any external postcombustor for N.sub.2 O reduction.
By introducing additional fuel, such as natural gas, in the flue gas passage in front of the convection section where the temperature of the flue gas still is high, only a relatively small amount of additional fuel is needed to increase the temperature of the flue gas flow to over 900.degree. C.
A cyclone separator may provide very good mixing of flue gases and any additive introduced therein. It may, however, be more preferable to increase the temperature of the flue gases at a location downstream of the particle separator (at least in circulating fluidized bed systems) in order not to increase the temperature of fluidized bed particles and interfere negatively with sulphur capture (which is optimal at lower temperatures).
The introduction of additional fuel into the flue gases may be advantageously used to increase the temperature of the flue gases upstream of superheaters, thereby ensuring sufficient heating capacity. The fuel may be added into a convention section immediately before the superheaters. The introduction of combustible additives may also be used to simultaneously increase the temperature of gas in a combustion chamber or so called topping combustor connected to a gas turbine.
When additional fuel is introduced into the flue gas flow before the convection section, the temperature of the flue gas has to be only moderately increased from temperatures of about 700.degree.-900.degree. C. to temperatures of about 910.degree.-1100.degree. C., i.e. a temperature increase of about only 10.degree.-250.degree. C. is enough, because of the presence of particles (e.g. calcined limestone) from the fluidized bed. If the flue gases pass through a convection section, their temperature is substantially reduced. Therefore, if the N.sub.2 O reduction is performed after the convection section, the temperature of the flue gases must be raised about 200.degree.-700.degree. C. in order to get it into the 910.degree.-1100.degree. C. range. Therefore, the amount of fuel necessarily added after a convection section is greater than the amount necessary before a convection section.
By using this process according to the invention to increase temperature and/or H radical concentration in the flue gases it may be possible to reduce the total amount of N.sub.2 O by 10-99%, normally about 50%, and preferably about 50-90%. The mass flow of the additive is defined by the percentage of N.sub.2 O reduction required and the initial concentration of N.sub.2 O.
In addition to the additive (e.g. additional fuel) injected, a suitable amount of oxidizing agent may in some cases be injected into the N.sub.2 O-containing flue gas before, at the same location, or after fuel injection point to guarantee efficient firing.
The present invention provides a method, which brings about conditions favorable to reduction of N.sub.2 O in flue gases in fluidized bed combustors, and thus a simple way of reducing N.sub.2 O emissions in flue gases. The new method can easily be utilized in existing fluidized bed reactor systems by introducing an additive into flue gas ducts, before stack or gas turbines or into external postcombustors. There is no need to interfere with the primary combustion process or the reactions taking place in the combustion chamber itself. Surprisingly, only a very slight increase in temperature may be needed for the reduction of N.sub.2 O in the flue gases. Prior art studies indicate destruction of N.sub.2 O in the furnace itself and at much higher temperatures. The increased temperature helps to promote destruction of N.sub.2 O not only by H-radicals in the gas phase but also the heterogenous reaction between N.sub.2 O and calcined limestone. Prior art studies show that N.sub.2 O formation reaches a maximum at the very temperatures at which N.sub.2 O is destroyed according to the present invention.